1. Field of the Invention
This invention relates to curable polyorganosiloxane compositions. More particularly, this invention relates to improved fluorinated polyorganosiloxane compositions that cure by a hydrosilation reaction to yield gels exhibiting a resistance to hardening when heated.
2. Description of the Prior Art
The term "polyorganosiloxane gel" typically defines a class of elastomeric, readily deformable materials exhibiting a surface hardness of below about 20 on the Shore 00 durometer scale. The hardness value of many gels is too low to be measured on this scale, and is often expressed as the depth to which a penetrometer probe of specified dimensions will penetrate or deflect the surface of the gel under a specified loading.
U.S. Pat. No. 4,719,275, which issued to K. Benditt and M. Maxson on Jan. 12, 1988 is directed to organosiloxane compositions that cure to yield a clear, thermally stable fluorosilicone gel. The compositions comprise (1) a liquid diorganovinylsiloxy terminated polydiorganosiloxane wherein at least 25 percent of the silicon atoms are bonded to a fluorine-containing hydrocarbon radical (2) a compatible liquid fluorine-containing organohydrogensiloxane having an average of more than two silicon bonded hydrogen atoms per molecule in an amount sufficient to provide a molar ratio of silicon bonded hydrogen atoms to vinyl radicals in said composition of from 0.5 to about 1.0; and (3) an amount of a platinum-containing catalyst sufficient to promote curing of said composition.
The polydiorganosiloxane referred to as ingredient (1) is prepared by the ammonia-catalyzed polymerization of at least one cyclic diorganosiloxane followed by reaction of the resultant liquid silanol terminated polydiorganosiloxane with at least a stoichiometric amount of a hexaorganodisilazane wherein a vinyl radical is bonded to each of the two silicon atoms.
The unique physical and chemical properties of the polyorganosiloxane gels disclosed in the aforementioned patent to Benditt and Maxson make them desirable as coatings and encapsulants for electrical and electronic devices to protect these devices from moisture, other contaminants and mechanical abuse, all of which can cause the device to malfunction or become inoperative.
Electronic components associated with electronic ignition and emission control systems of automobiles and other vehicles powered by internal combustion engines are often located in the engine compartment of the vehicle where they are exposed to temperatures of 100.degree. C. and above in addition to hydrocarbon fuels and lubricating fluids. Many prior art polyorganosiloxane gels harden or soften to the extent that they no longer provide the desired protection.
U.S. Pat. No. 4,122,246, which issued to Sierawski on Oct. 24, 1978, teaches preventing the discoloration of gels during aging. The composition for preparing the gel contains (1) a polyorganosiloxane containing an average of about two vinyl radicals per molecule and an average of from 2 to 2.03 hydrocarbon or fluoroalkyl radicals per silicon atom; (2) an organosilicon compound having an average of at least 3 silicon-bonded hydrogen atoms per molecule; (3) a polysiloxane having at least one silicon-bonded hydroxyl radical per molecule, at least two silicon-bonded vinyl radicals per molecule, and an average of less than 15 silicon atoms per molecule; and (4) a silane having at least one silicon-bonded epoxy-substituted hydrocarbon radical, at least one silicon-bonded alkoxy group having less than 5 carbon atoms per group; and (5) a platinum catalyst. The combined weights of ingredients (3) and (4) are less than 1.5 weight percent of the composition and the molar ratio of silicon bonded hydrogen atoms in (2) to vinyl radicals in (1) is less than 1. To achieve the desired resistance to discoloration ingredients (1), (3), and (4) must be combined before being blended with the remaining ingredients of the composition.
The gel obtained by curing the composition disclosed in the aforementioned Sierawski patent exhibits a penetration of from 2 to 60 millimeters measured using a Precision Universal Penetrometer.
Following the teaching of Sierawski to prepare a gel that is resistant to discoloration upon aging is less than desirable because of the two additional ingredients required, which increases the cost of the curable composition. In addition Sierawski does not address the problem of hardening and surface deformation observed when the fluorinated polyorganosiloxane gels disclosed in the aforementioned Benditt and Maxson patent are heated at temperatures of 100.degree. C. and above.
The use of dibutyltitanium acetylacetonate and ferric acetylacetonate as heat stabilizers for polydimethylsiloxane is taught in an article by Rafikov et al. that appeared in the Proceedings of the Academy of Sciences of the USSR [Volume 171, No. 6, page 1352 (1966)]. Decomposition of the polymer to hydrogen, methane and ethane was inhibited.
U.S. Pat. No. 4,528,313, which issued to T. Swihart and J. Jones on July 9, 1985 teaches using mixtures of iron acetylacetonate, nickel acetylacetonate and cerium acetylacetonate to impart thermal oxidative stability and increase the gelation time of a trimethylsiloxy-terminated polydimethylsiloxane fluid that is useful as a high temperature lubricant. The compositions optionally include a carboxylic acid containing from 2 to 9 carbon atoms.
One objective of this invention is to provide improved fluorine containing polyorganosiloxane compositions that upon curing yield gels exhibiting a resistance to discoloration, hardening and surface deformation when heated, and to do so without requiring reactants other than a vinyl containing polyorganosiloxane, an organohydrogensiloxane and a curing catalyst.